Pulse spectrometer employing sequences of synchronized modulated pulses

ABSTRACT

Pulse spectrometer, preferably for measurement of nuclear resonance of a material, having one or many high frequency measuring channels or having variable high frequency measuring ranges, characterized in that all pulse spacing periods within the pulse measuring programs and the HF carrier frequency of the HF pulses employed are derived from a common source of high frequency oscillations, so that the pulses commence only at specific, i.e. statistically nonfluctuating, phase values of the HF carrier frequency.

United States Patent lnventor Appl. No. Filed Patented Priority PULSESPECTROMETER EMPLOYING SEQUENCES OF SYNCHRONIZED MODULATED PULSES 15Claims, 1 Drawing Fig.

[1.8. CI 324/05 Int. Cl G0ln 27/78 Field of Search 324/05 [56]References Cited OTHER REFERENCES W, G. Clark Pulsed Nuclear ResonanceApparatus Rev. ofSci. Instr. 35(3), March 64, pp. 3 16- 333 PrimaryExaminerMichael J. Lynch Attorney-Craig, Antonelli, Stewart & HillABSTRACT: Pulse spectrometer, preferably for measurement of nuclearresonance of a material, having one or many high frequency measuringchannels or having variable high frequency measuring ranges,characterized in that all pulse spacing periods within the pulsemeasuring programs and the HF carrier frequency of the HF pulsesemployed are derived from-a common source of high frequencyoscillations, so that the pulses commence only at specific, i.e.statistically nonfluctuating, phase values of the HF carrier frequency.

' //L A, I 70 J. ,0 22 5 i i I I g E cmcun i /4 J! 5 I /.i I l 1 /Z V6 iINTEGRATOR i i ffff M i ll g' 22,270" 77 747 E car. if g suauumpurn I lI PULSE SPECTROMETER EMPLOYING SEQUENCES F SYNCHRONIZED MODULATED PULSESThe invention relates to a pulse spectrometer, particularly formeasuring nuclear resonances, with a transmitter for the production ofhigh frequency pulse sequences, the carrier frequency of which can beadjusted to one or several fixed values or can be constantly tunedwithin certain ranges.

Pulse spectrometers are employed, inter alia, for the spectroscopy ofnuclear resonances. Atomic nuclei with a magnetic moment other than zeroexecute a Larmor precession in an external magnetic field and exhibit amacroscopic magnetization M. In case of nuclear resonance, thismagnetization is altered, due to excitation of the nuclei, by analternating field, the frequency of which is in the range of theprecession frequency. This change in the macroscopic magnetization M ismeasured, for example, with the aid of an induction coil By the use of apulse-shaped field of excitation, the reactions of the nucleus to thisfield can be clearly separated from the effects of this field proper.

A pulse spectrometer suitable for such measurements, i.e., one whichproduces high frequency pulse sequences for the excitation of thenuclei, is described in Handbuch der Physik (Manual of Physics),published by S. Fluegge, Berlin, Goettingen, Heidelberg, 1958, Vol. 38/l, in the contribution by G. Laukien on pages 216 et seq. ln oneembodiment of the conventional pulse spectrometer, a high frequencytransmitter is directly excited by a pulse generator. In this case, thephases of the individual high frequency wave trains are scattered, sothat there-is no coherence whatsoever between the individual wavetrains. Therefore, this mode of pulse generation is called incoherent.In addition to exhibiting this deficiency in coherence, this mode ofoperation has the disadvantage that directly modulated oscillatorsexhibit a poor frequency stability. This disadvantage, though, isavoided in another construction of the conventional pulse spectrograph,wherein the output signal of a continuously operating oscillator ispulse-shape modulated. In this case, a satisfactory frequency stabilityand a phase coherence of the oscillations in the individual highfrequency pulses are attained; however, here the high frequency exhibitsa statistically varying initial phase with respect to the position ofthe ascending flank of the pulses. Consequently, the pulse widths andthe variations of the angles of rotation of the magnetization dependenton the pulse width also fluctuate statistically. In the last-mentionedembodiment of the conventional arrangement, the pulses serving formodulation purposes are produced with the aid of several independentmultivibrators; by means of these multivibrators, the pulse widths andpulse intervals can be set, sothat no relationship exists between thephase of the high frequency oscillation and the ascendlng flanks of thepulses.

The invention is based on the problem of avoiding these disadvantages ofthe conventional pulse spectrometer, and to provide a pulse spectrometerwherein, with a high frequency stability and phase coherence, the highfrequency oscillation always exhibits the same phase relationship withrespect to the initial flank of the pulse.

This problem is solved, in accordance with the invention, by providing ahigh frequency source of base oscillation, from which are derived boththe carrier frequency and the duration and spacings of the pulses of thepulse sequence. Due to this manner of derivation, there is also obtaineda fixed dependence between the high frequency carrier frequency and thepulses derived from the base frequency, so that the phase of the carrierfrequency is the same at the onset of each pulse.

in one embodiment of the invention, the source of base frequency isconstituted by a quartz-controlled oscillator. In place thereof, it isalso possible to employ an oscillator which can be tuned continuously orin stages, in particular a frequency synthesizer.

In a preferred embodiment of the invention, the carrier frequency isderived from the base frequency with the aid of frequency multiplierstages, but here again, a frequency synthesizer could also be employedfor this purpose.

As mentioned above, high frequency pulse sequences are produced in thepulse spectrometer of this invention, which sequences consist ofindividual pulses or of practically endless pulse sequences, but in somecases also ofa specific number of individual pulses with predeterminedwidths and varying spacings (see, in this connection, also HansStrehlow, Berichte der Bunsen-Gesellschaft [Reports of the BunsenSociety], Vol. 67, 1963, p. 255). The construction of the pulsespectrometer in accordance with the invention makes it possible in aparticularly simple manner to produce pulse sequences in accordance witha predetermined program. For this purpose, the provision is made, in afurther development of the invention, that a submultiplier (divider ordivider clock) is connected to the source of base frequency for theproduction of timing signals, which latter determine the duration andspacing of the pulses in the pulse sequence. This submultiplier can be achain of binary and/or decadic dividers, at the outputs of which theindividual time signals can be derived. All of these time signals areaccordingly derived from frequencies synchronous with respect to thebase frequency, and thus also sychronous with respect to one another. Inorder to form the pulse program, the time signals are preferablyselected with the aid of a logic selection circuit connected to thesubmultipler, and the timing signals necessary and selected for aspecific pulse sequence are fed to a pulse shaping stage.

The logic selection circuit allows only part of the timing signals topass through, which then serve as trigger signals for switching thepulse shaping stage on and/or off. Then, a gate circuit canadvantageously be connected to the pulse shaping stage; the carrierfrequency is fed to this gate circuit, and the gate circuit isperiodically opened by the signals produced by the pulse shaping stagein order to produce the high frequency pulse sequence.

The time signals supplied by the pulse shaping stage can also beemployed for controlling the mode of operation of the pulse spectrometerof the invention. Thus, one embodiment of the invention provides that anadditional logic selection circuit is connected to the submultiplier,with the aid of which timing signals produced by the submultiplier areselected and fed to the first logic selection circuit and the outputunits for the starting, terminating and repetition of a pulse program.Furthermore, the pulse shaping stage can produce control pulses foropening and blocking the measuring receiver of the pulse spectrometer,and these pulses can be fed to the measuring receiver.

In case of pulse spectrometers for the measurement of nuclearresonances, the carrier frequency is, for example, 60 or megacycles. Theexternal magnetic field, wherein the sample to be subjected tospectroscopy is disposed, is selected in such a manner in view of thecarrier frequency employed that nuclear resonance takes place. Ingeneral, the high frequency pulses fed to the sample exhibit arectangular definition and a length in the range of from I to 10microseconds.

The invention will be described and explained in greater detail belowwith reference to the embodiment illustrated in the drawing. The drawingshows a schematic block diagram of a pulse spectrometer in accordancewith the invention.

The pulse spectrometer shown as an example in the drawing includes abase frequency source or clock 10, constituted, for example, of aquartz-controlled base oscillator or a frequency decade circuit orsynthesizer; from the frequency of this source 10, the modulating pulsesas well as the high frequency carrier of the high frequency pulsesnecessary for the spectroscopy procedure are derived. This basicfrequency source 10 provides an output, for example, having a frequencyof l megacycle. in order to produce the high frequency carrierfrequency, the output signal of the base oscillator 10 is fed to a highfrequency generator 11, for example a multiplier or a frequency decadecircuit, which increases the frequency of the base oscillator to, forexample, 30 megacycleso In order to produce the modulating pulses, asecond output of the base frequency source 10 is connected toasubmultiplier 12 provided with binary and/or decadic divider stagesproducing a time signal system in the range of l microsecond to 100seconds with the stages 2, 4, 8 and 10 in each decade. With the aid of alogic selection circuit 13 connected to the submultiplier 12, those timesignals can be selected from the time signals delivered by thesubmultiplier 12 which are necessary for a desired pulse program. Inthis manner it is possible to select the various pulse sequence programsfor the respectively desired experiments, and to selectively composethese programs. The thus selected time signals are then fed to a pulseshaping stage 14 which shapes the time signals into rectangular DCpulses of a selectable duration. Then, the desired pulse program appearsat the output of the pulse shaping stage 14, which program can beutilized for the modulation of the high frequency carrier generated bysource 11. The base oscillator 10, the submultiplier 12, the logicselection circuit 13 and the pulse shaping stage 14 consequentlyrepresent a program generator.

The transmitter serving for the production of high frequency pulsesrequired for the pulse spectroscopy comprises, in addition to the highfrequency generator 11, a gate circuit 15 to which is fed the highfrequency carrier signal from the output of the high frequency generator11, and which gate circuit is, furthermore, controlled with the aid ofthe pulses derived from the pulse shaping stage 14 in such a manner thatthis gate circuit is open during the duration of these pulses anddelivers high frequency pulses at the output having the frequencyproduced by the high frequency generator 11. Thus, these pulses areeffective as a modulation of the HF carrier frequency. As a result, highfrequency pulse groups of a selectively determined duration and withselectively determinable spacings therebetween leave the gate circuit 15and are fed to the sample. In this connection, it is of importance thatall pulses exhibit identical phase relationships.

The high frequency pulses from switching circuit 15 exhibit the samevalues with respect to the phase relationship of the high frequency atthe ascending flank, and the high frequency is phase-coherent. The phasecoherence results from the fact that the high frequency carrier of thepulses is formed by parts of a constant, high frequency oscillation,whereas the fixed phase relationship of the high frequency pulsesresults from the fact that the high frequency carrier oscillation aswell as the time signals determining the onset of the high frequencypulses are derived from the same base oscillation, and accordingly allpulse intervals are integral multiples of the cycle of a frequency, theintegral multiple of which is the high frequency carrier frequency. Thisfrequency generally lies below the base frequency.

The high frequency pulses in the form of successive pulse groupsappearing at the output of the gate circuit 15, which pulses areselected in accordance with a specific program by the logic selectioncircuit 13, are fed optionally through a multiplier 17 to a sample head19 by way of a power amplifier 18; this sample head 19 is disposed in amagnetic field between the poles 20 of an electromagnet, the field coils21 of which are supplied by a power supply unit 22. From the highfrequency pulse sequence altered by the sample, modified pulse sequencesare derived, which sequences are fed, by way of a preamplifier 23, tothe main amplifier 24 of a receiver.

In case a heterodyne reception is to take place, it is possible to feedto the main amplifier the pure carrier frequency from the transmitter,which carrier frequency is contained in the selected pulse program. Forthis purpose, an additional multiplier 17' can be connected to the highfrequency generator 11 of the transmitter with the aid ofa switch 25;this multiplier corresponds to the multiplier 17 of the channel for thepulse sequence leading to the power amplifier 18 and the sample 19. Thehigh frequency oscillation serving as the reference or beat oscillationpasses from the multiplier 17, by way of an amplifier 26 and anadditional switch 27, to the main amplifier 24. The output signal of themain amplifier 24, which is converted to an intermediate frequency, canbe fed, for example, directly to an oscillograph 28; to the latter arefed calibration markers directly from the submultiplier 12 by way of theline 29, and trigger signals by way of a further logic selection circuit34 and the line 35.

Depending on the type of information to be obtained with the aid of thepulse spectrometer, the output signal of the main amplifier 24 can alsobe rectified by means of a phase detector 30 or a diode circuit 31 andthen indicated on the oscillograph 28 after rectification step, orrecorded in some other manner. Thus, the integrator 32, for example,makes it possible to obtain a selective integration which, in turn,makes it possible, for example, to select from a descending curve orfrom a succession of echos the measuring points which are actually ofinterest, for example, the maximum echo heights. The contour of asuccession of echos can be recorded, for instance, with the aid of arecorder 33.

As mentioned above, the submultiplier 12 yields a vary of timingsignals, from which predetermined timing signals can be selected withthe aid of the logic selection circuit 13. These timing signals aredigital signals, so that the logic selection circuit can he formed alsofrom digital circuits, and there is the possibility to automatize themode of operation of the pulse spectrometer of this invention, with theuse of conventional digital technique, by setting a program, accordingto which the pulse spectrometer executes experiments. In thisconnection, it is possible to repeat the same pulse sequences severaltimes,

or to constantly vary the pulse sequences according to a specificprogram. The trigger signals required for this purpose are derived, inthe illustrated embodiment, from the additional logic selection circuit34 and fed, via line 36, to the first logic selection circuit 13. Themeasuring results which are repeated several times in such an automaticprocess can be recorded one above the other, for example, for decreasingthe noise level, or they can be electronically evaluated by means ofsuitable evaluating devices which are not illustrated in detail.

The pulse spectrometer illustrated as an embodiment furthermore affordsthe possibility to vary the phase relationship and/or the frequency ofthe carrier signal from pulse to pulse in a pulse sequence. For thispurpose, several high frequency channels are provided between the basefrequency source 10 and the gate circuit 15, each channel containing ahigh frequency generator 11 and 11 as well as 11 and furthermore a phaseshifter 16 and 16' as well as 16' With the aid of the high frequencygenerators 11 to 11", it is possible to produce in the individualchannels selectively either identical or differing carrier frequencies.Besides, by means of the phase shifters 16 to 16', the phaserelationship of the high frequency oscillation with respect to thepulses fed to the gate circuit 15 is adjusted so that it is phase-lockedin each channel. With the aid of the logic selection circuit 13, thepulse shaping stage 14 and the gate circuit 15 it is then possible toselect from each of the high frequency channels any desired number ofpulses in any desired sequence, in order to compose them into a desiredpulse program. Of particular interest are, for example, pulse sequenceswherein successive pulses, though having the same carrier frequency,differ in phase relationship by specific amounts, for example or In thiscase, the high frequency generators 11, l1 and 11" are set to the samefrequency, and the desired phase difference is obtained with the aid ofthe phase shifters 16 to 16".

As an example, for a pulse sequence which can be selected in the pulsespectrometer of the invention, the Carr-Purcell pulse sequence is to bementioned, consisting of a first pulse of the duration 1 and a number ofadditional pulses of the duration 2!. The spacing between the firstpulse of the duration t and the first pulse of the duration 2t is A,whereas spacing between the pulses with the duration 2t is 2A. Thispulse sequence is of advantage for a number of experiments. Thissequence could be produced, for example, by deriving from thesubmultiplier 12 equidistant timing signals of an alternatingly positiveand negative polarity, of which the first positive time signal finds anopen gate in the logic selection circuit 13, which gate is closed bythis signal after passage therethrough, whereas the negative pulses areall allowed to pass through. In the subsequent pulse shaping stage, thefirst and only timing signal which has passed the gate then produces apulse of the duration t, whereas the subsequent negative timing signalsproduce pulses of the duration 2:.

With the aid of the program generator of the pulse spectrometer of thisinvention, it is also possible to produce other pulse sequences, suchas, for example, triplet sequences which can consist, for instance, of astarting pulse and a succession of triplets, each of these tripletscomprising two equally long pulses and a longer pulse positioned in themiddle. In addition to the timing signals for producing the desiredpulse sequence, as well as the timing signals and carrier signals forthe oscillograph 28, the integrator 32 and the recorder 33, it is alsopossible to derive from the base oscillator and the submultiplier 12further control pulses. For example, it is possible to actuate the mainamplifier only when an interesting bit of information is to be expected.

I have shown and described one embodiment in accordance with the presentinvention. It is understood that the same is not limited thereto but issusceptible of numerous changes and modifications as known to a personskilled in the art and I, therefore, do not wish to be limited to thedetails shown and described herein, but intend to cover all such changesand modifications as are obvious to one of ordinary skill in the art.

I claim:

1. A pulse spectrometer, preferably for measuring the nuclear resonanceof a sample, comprising:

a primary source of RF oscillations,

means for producing an RF carrier signal in synchronism with said RFoscillations,

program control means, responsive to said RF oscillations produced bysaid primary source, for generating a consecutive series of pulse groupshaving selectively variable spacing and length,

synchronizing means coupled to said primary source of RF oscillationsand to said program control means for synchronizing the pulses of saidpulse groups with said RF oscillations,

transmitter means connected to said means for producing an RF carriersignal and said program control means for modulating said RF carriersignal by said pulse groups to produce a series of pulse modulatedsignals, and

magnetic testing means for applying said pulse modulated signals to saidsample and for detecting nuclear resonances produced therein.

2. A pulse spectrometer as defined in claim ll wherein said primarysource is a crystal controlled base oscillator.

3. A pulse spectrometer as defined in claim 1 wherein said primarysource is an oscillator which can be tuned continuously.

4. A pulse spectrometer as defined in claim 1 wherein said programcontrol means includes a frequency submultiplier connected to saidprimary source of generating output signals of selectively variableduration and rate to form said pulse groups.

5. A pulse spectrometer as defined in claim 4 wherein said means forproducing an RF carrier signal includes a multiplier connected to saidprimary source for producing said RF carrier signal and said transmittermeans includes gating means connected to said multiplier and responsiveto said output signals for gating said carrier signal to form said pulsemodulated signals.

6. A pulse spectrometer as defined in claim 1, wherein said means forproducing an RF carrier signal includes a frequency synthesizer forproviding said RF carrier signal from which said pulse modulated signalsare derived.

7. A pulse spectrometer as defined in claim 1 wherein said programcontrol means includes a series of frequency dividers connected to saidprimary source for providing signals whose frequencies are mutuallysynchronous with the frequency produced by said primary source forselective application to said transmitter means. I

8. A pulse spectrometer as defined in claim 7 wherein the outputs ofsaid frequency dividers are selectively connected to first logic circuitmeans for permitting only selected ones of the pulses derived from thedivided frequencies to pass in accordance with a given program, andpulse shaping means connected to said logic circuit means for shapingthe pulses derived therefrom.

9. A pulse spectrometer as defined in claim 8, wherein said transmittermeans includes gating means connected to said pulse shaping means andresponsive to said pulses for gating said RF carrier signal to form saidseries of pulse modulated signals.

10. A pulse spectrometer as defined in claim 9 further includingadditional logic circuit means connected to said frequency dividers forproviding control signals to start, ter minate and repeat a program insaid first logic circuit means.

11. A pulse spectrometer as defined in claim 1 wherein said means forproducing an RF carrier signal includes a plurality of high frequencychannels connected to said primary source and said transmitter meansincludes gating means responsive to said pulse groups produced by saidprogram control means connected to said channels for gating said carriersignals in selectable succession.

12. A pulse spectrometer as defined in claim llll wherein said highfrequency channels each pr-ovide carrier signals of different frequency.

13. A pulse spectrometer as defined in claim 11 wherein said highfrequency channels each provide carrier signals having a differentphase.

14. A pulse spectrometer as defined in claim 11 wherein each highfrequency channel includes: at least one multiplier an done phaseshifter.

15. A pulse spectrometer as defined in claim 1 wherein said primarysource is provided in the form of a frequency synthesizer.

1. A pulse spectrometer, preferably for measuring the nuclear resonanceof a sample, comprising: a primary source of RF oscillations, means forproducing an RF carrier signal in synchronism with said RF oscillations,program control means, responsive to said RF oscillations produced bysaid primary source, for generating a consecutive series of pulse groupshaving selectively variable spacing and length, synchronizing meanscoupled to said primary source of RF oscillations and to said programcontrol means for synchronizing the pulses of said pulse groups withsaid RF oscillations, transmitter means connected to said means forproducing an RF carrier signal and said program control means formodulating said RF carrier signal by said pulse groups to produce aseries of pulse modulated signals, and magnetic testing means forapplying said pulse modulated signals to said sample and for detectingnuclear resonances produced therein.
 2. A pulse spectrometer as definedin claim 1 wherein said primary source is a crystal controlled baseoscillator.
 3. A pulse spectrometer as defined in claim 1 wherein saidprimary source is an oscillator which can be tuned continuously.
 4. Apulse spectrometer as defined in claim 1 wherein said program controlmeans includes a frequency submultiplier connected to said primarysource of generating output signals of selectively variable duration andrate to form said pulse groups.
 5. A pulse spectrometer as defined inclaim 4 wherein said means for producing an RF carrier signal includes amultiplier connected to said primary source for producing said RFcarrier signal and said transmitter means includes gating meansconnected to said multiplier and responsive to said output signals forgating said carrier signal to form said pulse modulated signals.
 6. Apulse spectrometer as defined in claim 1, wherein said means forproducing an RF carrier signal includes a frequency synthesizer forproviding said RF carrier signal from which said pulse modulated signalsare derived.
 7. A pulse spectrometer as defined in claim 1 wherein saidprogram control means includes a series of frequency dividers connectedto said primary source for providing signals whose frequencies aremutually synchronous with the frequency produced by said primary sourcefor selective application to said transmitter means.
 8. A pulsespectrometer as defined in claim 7 wherein the outputs of said frequencydividers are selectively connected to first logic circuit means forpermitting only selected ones of the pulses derived from the dividedfrequencies to pass in accordance with a given program, and pulseshaping means connected to said logic circuit means for shaping thepulses derived therefrom.
 9. A pulse spectrometer as defined in claim 8,wherein said transmitter means includes gating means connected to saidpulse shaping means and responsive to said pulses for gating said RFcarrier signal to form said series of pulse modulated signals.
 10. Apulse spectrometer as defined in claim 9 further including additionallogic circuit means connected to said frequency dividers for providingcontrol signals to start, terminate and repeat a program in said firstlogic circuit means.
 11. A pulse spectrometer as defined in claim 1wherein said means for producing an RF carrier signal includes aplurality of high frequency channels connected to said primary sourceand said transmitter means includes gating means responsive to saidpulse groups produced by said program control means connected to saidchannels for gating said carrier signals in selectable succession.
 12. Apulse spectrometer as defined in claim 11 wherein said high frequencychannels each provide carrier signals of different frequency.
 13. Apulse spectrometer as defined in claim 11 wherein said high frequencychannels each provide carrier signals having a different phase.
 14. Apulse spectrometer as defined in claim 11 wherEin each high frequencychannel includes at least one multiplier an done phase shifter.
 15. Apulse spectrometer as defined in claim 1 wherein said primary source isprovided in the form of a frequency synthesizer.